Kazuyuki Okamoto

Structural Differences in Amylopectin Affect Waxy Rice Processing

Abstract We studied the varietal differences in the rice cake hardening rate, which is one of the important characters in waxy rice processing. Waxy upland rice cv. Kanto-mochil72 had fewer short chains (dp 7 to 10) of amylopectin and more intermediate-length chains (dp 12 to 21) than cv. Tsukuba-hatamochi and cv. Mangetsu-mochi, which harden much more slowly when made into rice cakes, mochi, than does Kanto-mochil72. Based on zymogram analysis of the soluble fraction from the developing endosperm, the activity of starch synthase 2 (SS2), a candidate enzyme responsible for the difference in the amylopectin chain length was detected with Kanto-mochil72. SS2 activity was absent, however, in Tsukuba-hatamochi and Mangetsu-mochi. Using the progeny derived from a cross between Kanto-mochil72 and Mangetsu-mochi, we showed that differences in amylopectin chain length corresponded to the gelatinisation temperatures of the rice flours and rice cake hardening rates. The progeny of those rich in short amylopectin chains showed lower gelatinisation temperature and slower rice cake hardening rate than the progeny of those having amylopectin with fewer short chains. These results indicate that the differences in chain length distribution of amylopectin, polymorphisms in starch synthase activity, differences in rice flour gelatinisation temperature, and differences in rice cake hardening rate are not merely correlated with each other but are intimately linked since they are related to amylopectin properties.

Characterization of Waxy Rice Cakes (Mochi) with Rapid Hardening Quality by Instrumental and Sensory Methods

ABSTRACT The textural properties of cooked waxy rice cakes made from four waxy rice varieties including the unique varieties Kantomochi 172 (K172) and BC3 with the property of rapid hardening were analyzed by instrumental and sensory methods. For the instrumental analysis, a compression test, adhesiveness test, and tensile test were conducted. The waxy rice cakes made from K172 and BC3 showed significantly higher compressive force and resistance to break under tensile load. Significant difference in amylopectin chain-length distribution was observed between each variety, and this difference strongly reflected the hardness of waxy rice cakes. The peak area ratio of amylopectin branch chains with 6–12 degrees of polymerization negatively correlated with the compressive force required for 50 and 80% strain. Sensory evaluation showed that the waxy rice cakes made from these varieties had a significantly harder, less stretchable, less smooth surface, whereas the scores for adhesiveness and ease to cut off (hag...

Comparison of Starch Physicochemical Properties of Waxy Rice Cultivars with Different Hardening Rates

The objective of this study was to investigate the starch characteristics of a novel waxy rice cultivar Hitachimochi 36 (H36) with an extremely slow hardening rate of waxy rice cake and the relationships between starch physicochemical properties and texture of waxy rice cake. Starch isolated from H36 showed significantly higher digestibility than that from other waxy rice cultivars, and the starch digestibility highly correlated with the starch crystallinity. The compressive force of the starch gel prepared from H36 was significantly lower than that from other cultivars when stored at 5°C for five days, which reflected the differences in endothermic enthalpies corresponding to retrograded amylopectin. Various textural parameters of cooked waxy rice cake prepared from H36 were also remarkably different from those of cooked waxy rice cake prepared from other waxy rice cultivars. The cooked waxy rice cake prepared from H36 exhibited the lowest compressive force and breaking force by tensile and rupture tests...